DN THE MICROSCOPIC STRUCTURE OP SHELIjS. 117 
considerable portion oF the thickness of the shell of the latter. My examina¬ 
tion has not yet been extended to the fossil shells of this class, with the ex¬ 
ception of the calcareous sheath of the Belemnitc; the appearances presented 
by which lead me (as already stated, § 85) to regard it as having originally 
had a structure similar to that of Sepiariu ffipantm. 
KW. in the caleifietl layers which alterimtc with horny momhranes to 
form the shallow cone or cup of the internal shell (or Sepiostaire) of Sepia^ 
a distinct cellular arrangement is visible, when the section is made sufficiently 
tliin. The delicate and fragile transverse plates which are arranged obliquely 
upon one another iu the hollow of this cup, also show indications of cellular 
structure, although these arc by no means diatiuet. The thin chambers be¬ 
tween these transverse plates are traversed by an infijjity of minute columnar 
and sinuous partitions, planted at right angles to the plates, and disposed 
as to cause the plates mutually to support each other- Professor Owen has 
spoken of these as “ crystalline fibres passing perpendicularly from one layer 
Jo the other" (Cyclopaedia of Anatomy and Physiology, vol. i. p. j 46) ; but 
if the surface of oue of the parallel plates be examined, it is found to be 
traversed by sinuous lines, which frequently return almost completely upon 
themselves; and these lines are evidently the base-lines of the vertical par* 
titioiis, which are rather plates than fibres, although very apt to break into 
fibres when an attempt is made to obtain a thin section of this curious struc¬ 
ture. They often prj-aent upon their surface a series of definite lines or 
ridges, running parallel to the trajisvcrsc pUitca which they serve to connect; 
these lines strongly remind us of the transverse .sti iic ujion the prismatic cells 
of Pinna, and are probably indicative of the same kind of cellular origin. 
105. The shell of Spirula must be considered (as is now uaiversally ad¬ 
mitted) to bear a greater resemblance, as regards its relation to the animal, 
to the Sepiostaire, than to the ehamberwl fihell of Nantilug, although it so 
closely approximates the latter in iu own ourifonuation, Thb beiug the case, 
ff 1.S interesting to find that the intimate structure of the shell has a much 
greater rcscrablance to that of the Sepiostaire than would be supposed from 
Jts general aspect. For although the texture of the shell seems uniform, and 
Its minute parts are composed of an aggregation of calcified cells, yet its 
nirface is marked by sinuous lines, closely resembling those which arc seen 
Jipon the transverse plates of the Sepiostaire; and these line^ or bands pro¬ 
ject in such a degree, that they might be considered as rudiments of the 
’Vertical partitions which connect those plates. 
XVII. Echimdemiala. 
106. The structure of the skeleton of this class is entirely different from 
that which we have found to be characteristic of the Mollusca, wiiilst in its 
Essential features it presents a remarkable uniformity throughout the various 
tnembers of the group. The general arrangement of its ctimponcnts is the 
fame, for cxauiple, in the firm plates which make up the testa of iXio-EckinidOy 
•n the joints of the stems and brauchea of tlui Crinvitha-, and in the 
•raftered calcareous deposits which are met with in the intogument 
and in the tonlacula of the HulvUivridu. A very elaborate and com- 
piete dracriptiou of the skidcton of tlie ErhinuH U given by Professor Valen¬ 
tin , his adnairublu luutiogvaph on that genus, which forms one of 
‘Monographies d’KchiuodBnn(>s” of Professor Aga.«ssiz; ami with bis 
investigations, so tar as they ndate to tlu! structure of tlie (cstn, my own are 
•D Complete accordance. I believe, liowevetr, that I am enabled to give a 
>wch more accurate account of the structure of the spines than he coulil 
“OKI, as ray method of preparing sections of them enables me to reduce 
